This invention relates to a method of controlling silica reinforcement in a curable silicone composition having a desired property related to the silica reinforcement, the method comprising adding to the curable silicone composition a volume fraction of reinforcing silica filler sufficient to provide the desired property, where the volume fraction is determined by use of a regression analysis equation relating the volume fraction, Carman surface area (CSA), and Bumauer-Emmet-Teller surface area (BET surface area) of the reinforcing silica filler to the desired property.
It is recognized in the art of elastomer reinforcement that filler properties play a major role in determining elastomer Theological and mechanical properties. In general, viscosity or plasticity of the uncured elastomer, and yield, stress, durometer, and modulus of cured elastomers increase with increasing BET surface area. An important reinforcing filler used in silicone elastomers is amorphous silica. Two types of amorphous silicas are used, fumed and precipitated, with fumed silica use predominating. Like other reinforcing fillers, silicas are typically specified by BET surface area. BET surface area is determined by the Bumauer-Emmet-Teller method, which is based on nitrogen adsorption, and is described in The Journal of the American Chemical Society, 60 p. 309 (February, 1938). Nitrogen adsorption measures the surface areas of the individual, spherical, primary particles of silica.
Although BET surface area can be quantitative predictor of performance of some non-silica reinforcing fillers in hydrocarbon-based elastomer, it is less useful in predicting the effect of silica reinforcing fillers on properties in either hydrocarbon-based or silicone elastomers. Machurat, et al., in U.S. Pat. No. 4,251,281 teach that silica particles having considerably different BET surface areas can give similar properties in organic elastomer. More recently, Okel and Waddell reported in Rubber Chemistry and Technology, 68 p. 59 (March-April 1995) that, for precipitated silica fillers used in high consistency silicone elastomer, no single physical property of the silica could be used to predict silicone elastomer performance. Similarly, in silicone elastomer formulations containing fumed silicas, different silicas that have the same BET surface area measurement can give different Theological and mechanical properties in silicone sealant formulations. Therefore, new methods of specifying silica characteristics are needed to improve the consistency of silicone elastomer properties.
The Carman surface area is a measure of the structure level of particles. The term structure, in this instance, refers to the tendency for spherical particles to form clusters or aggregates. The method for measuring Carman surface area (CSA) is described by H. Carman and P. Malherbe, J.Soc.Chem.Ind.,London 69, 134 (1950). A known weight of silica is compressed into a pellet of known dimensions and density. Air flow through the pellet is then measured. The reported CSA is the relationship between the air flow and the compressibility of the aggregates. Depending on the aggregate structure, voids of various sizes are formed within the pellet. These voids control the air flow through the silica. High structure fumed silicas have smaller, open branched chain aggregates and high Carman surface area values. For instance, a highly structured aggregate with many fingers will closely compact, forming small voids. In turn, the air flow rate will diminish, yielding a higher CSA. Low structure fumed silicas have a large grape cluster-like aggregates and low CSA values. Depending upon manufacturing conditions such as flame temperature and feed rate, a broad range of CSA values, and therefore aggregate shapes, can be prepared for a silica having a given BET surface area.
CSA has not previously been recognized as an important parameter in characterizing silicas for use as reinforcing fillers in silicone elastomers. Recently, however, H. Cochrane and C. S. Lin, in Rubber Chemistry and Technology 66(1) pp. 48-60 (1993), "The Influence Of Fumed Silica Properties On The Processing, Curing, And Reinforcement Properties Of Silicone Rubber," investigated the effect of varying fumed silica properties including load, surface area, silica structure level, and surface pretreatment levels, on the silicone elastomer processing, curing, and cured physical properties. Structure level was measured with by CSA. They found that increasing the silica structure level at a constant BET surface area gave elastomer compounds with increases in plasticity, creping, crosslink density, modulus yield point, and durometer.
The use of regression techniques to analyze data is well known and applied to various fields of science and engineering. Regression procedures use the values of one or more independent variables (the x values) to predict the value of a dependent variable (the y value). The independent variables are the known, or predictor, variables. When the independent variables are varied, they result in a corresponding value for the dependent, or response, variable.
To perform a regression analysis, data on properties for several known samples is collected and a regression analysis equation is generated that describes the relationship of the properties to each other. A multidimensional regression equation may or may not be graphically represented, and the regression equation may be non-linear. In these cases, the equation that provides the best fit for the data is typically determined by several iterations, and may be assisted by use of a computer program. Once the equation that describes the relationships between properties for known samples is determined, that equation can be used to predict unknown properties in new samples. The known properties of the new samples are used in the equation, and the equation can be solved for the value of the unknown properties. For example, Dechene et al. in U.S. Pat. No. 5,367,260 teach a method by which physical properties of thermoplastics are measured by NMR techniques and related back to the flow rates of those plastics.
The use of regression analysis of physical properties of silica to predict the performance of silicone rubber has not been studied extensively. However, Okel and Waddell, supra, did analyze 25 properties of precipitated silicas on fourteen silicone rubber performance characteristics. They concluded that, for precipitated silica, a combination of (BET) surface area, oil absorption, particle size, pH, adsorbed moisture, residual sodium and/or residual salt can qualitatively predict the physical performance of high-temperature-vulcanized, high-consistency, precipitated silica-filled silicone rubber.
The use of the silica parameters defined by Okel and Waddell, supra, to control properties in fumed silica-filled silicone rubber is not particularly practical. For fumed silica, properties such as residual sodium are not relevant because fumed silicas are not produced from the acid neutralization of a metal silicate in the manner that precipitated silicas are. Furthermore, the parameters only describe the characteristics of the silica, and do not provide any ability to adjust a silicone elastomer formulation to achieve desired silicone elastomer properties for a given silica.
An object of this invention is to provide a method of controlling a Theological or physical property of a given silicone elastomer composition of known silica volume fraction by selecting the BET and CSA of a silica. A further object of this invention is to provide a method for adjusting the amount of silica in a silicone elastomer formulation to provide a desired physical or rheological property when the CSA and BET are known.